Nickel-alumina aerogel catalyst for carbon dioxide reforming of methane and the preparation method thereof

ABSTRACT

The present invention relates to a method of containing nickel into the alumina aerogel prepared by sol-gel method and supercritical drying and of preparing the nickel-alumina hybrid aerogel catalyst. The nickel-alumina catalyst prepared in the present invention has an excellent reactivity with a prolonged lifetime.

BACKGROUND OF THE INVENTION

The present invention relates to a method of preparing highly reactivenickel catalysts with an increased lifespan that is used in preparing asynthetic gas by reforming methane, a main component in natural gas,with carbon dioxide. More particularly, the present invention relates toa method of preparing nickel-alumina hybrid aerogel catalyst by using asol-gel method or by having alumina aerogel to carry nickel thereon, ina different manner than that of the prior nickel catalyst supported onalumina.

Generally, an aerogel is prepared by supercritically drying a gel from asol-gel reaction. The aerogel has a specific high surface area ratiowith a low density. Due to its physical characteristics, aerogel isuseful as a catalyst itself or can be used as a carrier for a catalyst.Supercritical drying is a process of drying the gel above a criticalstate such as high temperature and high pressure to remove solvents heldin the gel structures and by this method, it is possible to preventshrinkage and crack formation due to capillary pressure differencesbetween the gas/liquid phase interface.

Carbon dioxide is considered to be relatively harmless. However, it hasbeen recently revealed as a major factor in global warming, and now itsproduction is restricted by climate control regulation. Therefore,people are now more interested in reducing carbon dioxide production orin developing physical and chemical methods to fix carbon dioxide.

One of the chemical methods in fixing the carbon dioxide is to produce amixture containing carbon monoxide and hydrogen by reforming methane, amajor component in natural gas, with carbon dioxide as shown below. Theproduced mixture can be utilized in generating a variety of chemicals.This is one of the more effective ways of utilizing the natural gas,which is used mainly as a fuel. It is practically useful to use methaneand carbon dioxide simultaneously, considering that methane has a higherglobal-warming tendency than carbon dioxide and when it is inconvenientto transport the methane, it is wasted by burning it in-situ, whichproduces undesirable carbon dioxide.

CH₄+CO₂→2CO+2H₂ (ΔH=261 kJ/mol)

The synthetic gas produced by the reaction above has a carbonmonoxide/hydrogen ratio of 1:1 and can be used as a starting materialfor many chemicals. The purity of the produced carbon monoxide is knownto be higher than by any other methods. And this reaction characteristicis similar to the conversion method using steam in producing hydrogengas, and therefore already established processes and knowledge in thefield can be utilized. A durable catalyst is urgently needed to bedeveloped since catalyst can be easily deactivated due to the cokingduring the reaction when the commercial nickel catalyst for the steamreforming is used.

According to the literature, the deactivation of the noble metalcatalyst is markedly slow [A. T. Ashcroft, A. K. Cheetham, M. L. H.Green, and P. D. F. Vernon, Nature, 225, 352 (1991)]. Considering theeconomical aspect, however, it is impractical to use the noble metalcatalysts when a large amount of catalyst is needed. Therefore, therehave been many studies to improve the durability of the nickel catalystby using a co-catalyst or additives.

In the commercialized SPARG process, nickel catalyst treated with sulfuris used to improve the durability. In this process, however, there aremany problems such as the sulfur must be supplied continuously to thereaction mixture and the reaction needs to be carried out at atemperature higher than 900° C. to improve the reactivity of thecatalyst [H. C. Dibbern, P. Olesen, J. R. Rostrup-Nielsen, P. B.Tottrup, and N. R. Udengaard, Hydrocarbon Process., 65, 71 (1986)]. Thedeactivation of the catalyst can be reduced by adding vanadium ormolybdenum into the nickel catalyst [T. Arakawa and M. Oka, U.S. Pat.No. 3,849,087], by adding a basic material such as calcium oxide [Z. L.Zhang and X. E. Verykios, Catal. Today 21, 589 (1994)] or by using basicmagnesium oxide that can contain the catalysts.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a durable catalystfree from deactivation problems due to coking. The catalyst of thepresent invention is useful in the process of simultaneous treatment ofmethane and carbon dioxide which are major contributors ofglobal-warming and is different from a prior nickel catalyst for steamreforming.

More particularly, it is an object of the present invention to provide ahighly reactive nickel catalyst with an increased lifespan that is usedin preparing a synthetic gas by reforming methane, a main component innatural gas, with carbon dioxide. That is, the present invention relatesto a method of preparing nickel-alumina hybrid aerogel catalyst by usinga sol-gel method or by having alumina aerogel to carry nickel thereon,in a different manner than that of the prior nickel catalyst supportedon alumina.

DETAILED DESCRIPTION OF THE INVENTION

One of the simplest processes for preparing alumina gel is thehydrolysis of the aluminum alkoxide using an acid catalyst and excessamount of water. In this case, it is required to exchange water withanother solvent such as alcohol to prepare the aerogel by supercriticaldrying [B. E. Yoldas, J. Mater. Sci., 10, 1856 (1975)]. In the case ofusing an organic solvent such as benzene or alcohol, it takes severaldays to synthesize gels even at high temperatures, and therefore, anadditive to accelerate the sol-gel reaction is often used [Y. Mizushimaand M. Hori, J. Non Cryst. Solids, 167, 1 (1994)]. However, in thesol-gel method of the present invention, a homogeneous alumina gel canbe synthesized at room temperatures without additives by slightlyaltering the solubilization, hydrolysis and condensation processes.

Detailed description of the preparation method of the alumina or thenickel-alumina hybrid aerogel by using the sol-gel method is as follows.

The preparation method of the alumina or the nickel-alumina hybridaerogel of the present invention is composed of: the step of dissolvingthe aluminum alkoxide in a heated alcoholic solvent; the step of forminga transparent sol by partial hydrolysis; the step of forming a gel atroom temperature; the step of aging the gel; the step of supercriticallydrying the gel; and the step of thermally treating the gel.

The first step of dissolving aluminum alkoxide involves adding aluminumalkoxide into a heated alcoholic solvent and vigorously stirring it fordissolution. The aluminum alkoxides represented by Al(OR)₃ (R is analkyl with 1 to 6 carbons), preferably aluminum isopropoxide or aluminumsec-butoxide, can be used for this process. Any kind of alcohol,preferably ethanol or 1-propanol, can be used. The ratio between thealcohol and alkoxide is preferably 0.4 to 0.6 mmol/ml, and thetemperature of the heated alcohol is preferably 40˜90° C.

The second step of forming a transparent sol by a partial hydrolysisinvolves adding the heated alcohol solution with aluminum alkoxide intoa mixture of acid, water and alcohol in a single step, and stirringwhile heating the solution continuously to form a transparent sol bypartial hydrolysis. There is no limit to the amount of alcohol used forthis step, however, it is preferable to use ⅓ to ⅔ of the amount thatwas used for the first step. It is also preferable to use water in amole ratio of 0.2 to 0.8 based on aluminum. It is preferable to use acidin a mole ratio of 0.01 to 1.0 based on aluminum. For instance, in thecase of nitric acid, it is preferable to use the nitric acid in theamount of {fraction (1/50)}˜{fraction (1/100)} in a molar ratio based onaluminum.

In the case of forming a nickel-alumina hybrid gel, completely dissolvedalcohol solution containing nickel as a salt form at a 0.03 to 0.3 ofmole ratio to the total amount of aluminum is added. There is no limitto the amount of alcohol used for this step, however, it is preferableto use 0.05 to 0.2 of the amount that was used for the step of formingthe transparent gel by partial hydrolysis. Any nickel salt thatdissolves well in alcohol, preferably nickel nitrate or nickel acetatecan be used.

The third step of forming a gel at room temperature involves cooling thetransparent sol prepared in the second step and of adding an alcoholthat contains a certain amount of water to form a transparenthomogeneous gel in a few minutes. It is preferable to use water in themole ratio of 0.5 to 1.5 based on aluminum.

The fourth step involves aging the gel formed at the third step for morethan 1 hour.

The fifth step of supercritical drying the gel involves incubating theaged gel at a higher than the critical pressure and temperature of thealcoholic solvent and changing the condition to room temperature atatmospheric pressure. Supercritical drying can also be performed byreplacing the solvent with carbon dioxide and by supplying carbondioxide at the supercritical condition as above to completely remove thealcohol to form an aerogel.

The sixth step of thermal treating the gel involves initially thermaltreating the gel at an inert atmosphere at 200 to 500° C. and asecondary thermal treating in air or oxygen atmosphere at higher than500° C. The thermally treated alumina aerogel can be used to as asupport for carrying the nickel. The nickel-alumina hybrid gel, whichwent through the second step of adding the transparent sol into thealcohol solution which contains the completely dissolved nickel salt andthrough the third through the sixth step, can be used as a catalyst forthe reforming reaction of methane with carbon dioxide. The increasingtemperature rate is 1 to 20° C. /min for all the processes.

As described above, a synthetic gas is produced by using the catalystsprepared in the present invention. In this case, by using a conventionalfixed-bed atmospheric flow reactor, first the hydrogen is supplied intothe catalyst bed, then the hydrogen supply is terminated and nitrogen isadded to purge the hydrogen. The methane and carbon dioxide reactantsare supplied with nitrogen continuously to produce the synthetic gas.The gas contact time in the catalyst bed during the pretreatment andreaction is 0.0005 to 0.005 g. min. l⁻¹, and the composition of thereactant is 15 to 30% methane, 15 to 30% carbon dioxide and nitrogenbalanced to 100%.

The invention will be further illustrated by the following examples, butthe present invention is not limited to the examples given.

EXAMPLE 1

12.32 g of aluminum sec-butoxide was added to 47.4 g (60 cc) of ethanolat 60° C. vigorously stirred to dissolve the mixture completely and themixed solution of 0.2044 g of 60% nitric acid , 31.6 g (40 cc) ofethanol and 0.45 g of water was added to form a transparent sol. Aftercooling this transparent sol to room temperature, a mixed solution of3.95 g (5cc) of ethanol and 1.0 g of water was added to form atransparent homogeneous alumina gel in 2 minutes. After the gel was agedfor 3 hours in closed state, the aged gel was transferred to a vesselfor supercritical drying. Then carbon dioxide was introduced at 60° C.,240 atm. for a supercritical drying to form an alumina aerogel. Theaerogel was treated for 2 hours at 300° C. at a 135 ml/min heliumatmosphere with a temperature increasing at a rate of 5° C. /min up to300° C. for the initial thermal treatment. The aerogel was then calcinedfor 2 hours at 500° C. after increasing the temperature with aincreasing rate of 5° C. /min for the secondary thermal treatment.

In order to carry the nickel on the prepared alumina aerogel, 10 g ofalumina aerogel was added into an aqueous solution containing 2.285 g ofnickel nitrate and 30 cc of distilled water and heated to 80° C. whilestirring well to obtain a slurry mixture. After drying the mixture in anoven at 120° C. for about 12 hours, a thermal treatment was performedfor 2 hours at 500° C. in a 95 ml/min oxygen atmosphere to form a nickelcatalyst contained in an alumina aerogel.

The reactivity of the catalyst was measured by using a fixed-bedatmospheric flow reactor. The catalyst (0.05 g) as prepared above wasput into a reaction tube and a pretreatment was performed for 2 hours at700° C. while hydrogen and nitrogen at 10 and 20 ml/min, respectively,were supplied. After the pretreatment, the hydrogen supply wasterminated and nitrogen was provided for another 5 minutes to purge thehydrogen before the reactants, methane and carbon dioxide, weresupplied. The flow rates of the gases were 10,10 and 20 ml/min,respectively, and the reaction temperature was 700° C. which wasidentical to the temperature during the pretreatment. The composition ofthe product was identified by using a gas chromatography, and theconversion of methane and carbon dioxide was calculated.

The result of the reactivity measurement of the nickel catalystcontained in alumina aerogel is summarized in Table 1. The conversion ofmethane and carbon dioxide was 68% and 70%, respectively, in 2 hoursafter the initiation of the reaction at 700° C. and decreased to 62% and64%, respectively, 30 hours after the reaction. The decrease in rate,however, was not high.

Since it is known that the decrease of the catalyst reactivity is mainlyfrom a coking, the carbon contents before and after the reaction weremeasured and the results summarized in Table 2. As can be seen in theTable, coking was not a problem since the carbon content was extremelylow even after 30 hours. Considering the carbon content in the aerogelitself, the deposited carbon during the reaction is lower than thelisted values in Table 2.

COMPARATIVE EXAMPLE 1

Using the alumina catalyst carrying nickel as in EXAMPLE 1 in aluminathat is conventionally used as a catalyst carrier, the reactivity of thereforming reaction of methane with carbon dioxide was measured. As canbe seen in Table 1, the conversions were 66 and 68%, respectively, in 2hours after the initiation of the reaction. After 3 hours, however, theconversion decreased dramatically coking. As can be seen in Table 2, thecarbon content 3 hours after the initiation of the reaction was 11.20%,which indicates severe coking.

COMPARATIVE EXAMPLE 2

Commercial catalyst from the Engelhard Co. (ESCAT44) that contains 5% ofa precious metal ruthenium in alumina was used to measure the reactivityat an identical condition as in EXAMPLE 1. As can be seen in Table 1,the conversion rates were 72% and 74%, respectively, in 2 hours and 71%and 72%, respectively, in 30 hours after the initiation of the reaction.The reactivity obtained using the precious metal catalyst is similar tothe results as in EXAMPLE 1.

EXAMPLE 2

Different from EXAMPLE 1, during the alumina formation by using thesol-gel method, nickel nitrate (Ni(NO₃)₂. 6H₂O) was added to form anickel-alumina hybrid aerogel as follows. In other words, as in EXAMPLE1, after a transparent sol was formed with ethanol, aluminumsec-butoxide, nitric acid and water, the temperature was decreased to60° C. before adding 7.9 g (100 cc) of ethanol solution containing 1.454g of nickel nitrate. After enough time had passed to form a homogeneoussolution, the temperature was decreased to room temperature, and 3.95 g(5 cc) of ethanol and 1.0 g of water were added to form a nickel-aluminahybrid gel. The prepared gel was supercritical-dried as in EXAMPLE 1,thermal-treated and used as a catalyst to measure the reactivity. Theresults of the reactivity and the carbon content measurements werelisted in Tables 1 and 2. As can be seen from the Tables, the reactivityis a little lower and the carbon content is a little higher than thosein EXAMPLE 1. However, the coking and deactivation of the catalyst wereconsiderably lower than those in COMPARATIVE EXAMPLE 1.

EXAMPLE 3

In preparing the catalyst as described in EXAMPLE 2, nickel acetate wasused as a nickel salt instead of nickel nitrate to prepare anickel-alumina hybrid aerogel. As can be seen in Tables 1 and 2, thiscatalyst has similar reactivity to the catalyst in EXAMPLE 2, whileshowing extremely low coking.

TABLE 1 Changes in conversion of methane according to the catalyst andthe reaction time COMPARA- COMPARA- Re- TIVE TIVE action EXAMPLE EXAM-EXAM- EXAM- EXAMPLE time 1* PLE 1 PLE 2 PLE 3 3 Methane conversion (%) 2 h 66 68 62 61 72 30 h — 62 52 47 71 Carbon dioxide conversion (%)  2h 68 70 64 66 74 30 h — 64 53 54 72 *Reaction was terminated in 3 hoursdue to plugging the reactor tube.

TABLE 2 Changes in carbon content according to the catalyst and thereaction time COMPARA- COMPARA- TIVE TIVE EXAMPLE EXAM- EXAM- EXAM-EXAMPLE 1* PLE 1 PLE 2 PLE 3 3 Before reaction — 0.30 0.51 0.54 — (700°C., hydrogen treatment only) After reaction 11.20* 0.97 2.08 0.78 0.25(700° C., hydrogen treatment and reaction)

The nickel-alumina catalyst prepared in the present invention was usedin the reforming reaction of methane with carbon dioxide. The catalystof the present invention is highly reactive with an increased lifespan.The sol-gel method used in the present invention is advantageous sincethe homogeneous gel can be obtained by simple methods in a short timeunlike the conventional alumina gel synthesis.

What is claimed is:
 1. A method of preparing an alumina aerogel supportfor a nickel-alumina catalyst comprising the steps of: a first step ofdissolving aluminum alkoxide in a heated alcoholic solvent; a secondstep of adding a mixture of water, acid and alcohol into the thuslyprepared solvent to form a transparent sol by partial hydrolysis; athird step of cooling the transparent sol to room temperature and thenadding alcohol containing water to the cooled sol to form a gel; afourth step of aging the gel; a fifth step of supercritical-drying theaged gel; and a sixth step of thermal-treating the dried gel initiallyin an inert atmosphere, and then in air or oxygen atmosphere.
 2. Themethod according to claim 1, wherein the temperature of the alcoholicsolvent in the first step is maintained at 40° to 90° C.
 3. The methodaccording to claim 1, wherein the aluminum alkoxide in the first step isaluminum isopropoxide or aluminum sec-butoxide.
 4. The method accordingto claim 1, wherein the aluminum/alcohol ratio in the first step is 0.4to 0.6 mmol/ml.
 5. The method according to claim 1, wherein the water toaluminum ratio in the second step is 0.2 to 0.8 to form the transparentsol.
 6. The method according to claim 1, wherein the water to aluminumratio in the third step is 0.5 to 1.5 to form a homogeneous gel in ashort time.
 7. The method according to claim 1, wherein the initialthermal treatment of the supercritically dried gel in the sixth step isperformed in an inert atmosphere at 200 to 500° C., and a secondarythermal treatment of the gel is performed at a temperature higher than500° C. in air or in oxygen.
 8. A catalyst composition comprising thealumina aerogel support material produced by the method of claim 1,wherein said catalyst comprises 1% to 40% of nickel and 60% to 99% ofalumina.
 9. A method for making a catalyst composition by producing asupport material according to the method of claim 1, wherein there is aseventh step of impregnating nickel salt on the thermally treated gelfrom the sixth step.
 10. A method of producing a nickel-alumina hybridaerogel catalyst comprising the steps of: a first step of dissolvingaluminum alkoxide in a heated alcoholic solvent; a second step of addinga mixture of water, acid and alcohol into the thusly prepared solventand a further step of adding an alcohol solution containing nickel saltinto the mixture to form a transparent sol by partial hydrolysis; athird step of cooling the transparent sol to room temperature and thenadding alcohol containing water to the cooled sol to form a gel; afourth step of aging the gel; a fifth step of supercritical-drying theaged gel; and a sixth step of thermal-treating the dried gel initiallyin an inert atmosphere and then in air or oxygen atmosphere.